Porous materials have been utilized in various fields such as adsorption and separation. According to IUPAC, porous materials are classified into microporous ones having a fine pore diameter of 2 nm or less, mesoporous ones having a fine pore diameter of 2 to 50 nm, and macroporous ones having a fine pore diameter of 50 nm or more. The microporous porous materials have conventionally been known and zeolites such as natural aluminosilicate salts and synthetic aluminosilicate salts, metal phosphate salts, and the like are known. They have been utilized for selective adsorption and form-selective catalytic reactions, and as molecular-size reaction vessels, in which the fine pore size of the fine pores is utilized.
As the mesoporous one, attention is attracted to mesoporous silica having a homogeneous pores with narrow pore size distribution in nanometer level are regularly arranged. It has a structure where the pores having an equal diameter are arranged in a honeycomb form. In comparison with the conventional porous materials such as zeolite, it has characteristics that the fine pore volume is large and the surface area is also large. Silica gel has a disordered porous structure and silica gel having such a high specific surface area and such a large pore volume has not been known.
In both cases, it is deemed that structural control of silica is achieved through an action of a surfactant aggregate as a template. These substances are not only very useful materials as catalysts for bulky molecules which do not enter the pores of zeolite, but also may be applied to functional materials such as optical materials and electronic materials by introducing guest species having various functions into the fine pores.
The mesoporous silica is specifically known as a material called MCM-41 synthesized by hydrolyzing alkoxides of silicon in the presence of a surfactant as described in Non-Patent Document 1 and a material called FSM-16 synthesized by intercalating an alkylammonium between layers of kanemite, which is one type of layered silicic acid, as described in Non-Patent Document 2.
Utilizing self-assembling of an organic compound and an inorganic compound, a porous substance having homogenous mesopores is produced. As the production process, for example, it is produced by hyrdothermal synthesis in a heat-resistant vessel in which silica gel and a surfactant are enclosed under tight sealing as described in Patent Document 1. Moreover, Non-Patent Document 2 describes a production process wherein it is produced by ion-exchange of kanemite, which is one type of layered silicate salts with a surfactant. Non-Patent Document 4 describes a process for producing an inorganic porous body by forming a three-dimensional highly regular composite of inorganic material-surfactant as a precipitate through hydrothermal synthesis using an aggregate of a surfactant consisting of an alkyltrimethylammonium as a template, subjecting the composite to solid-liquid separation and washing, and then removing organic substances contained therein through calcination. The concentration of the surfactant is determined to be a concentration higher than the critical micelle concentration and lower than the liquid crystalline phase-forming concentration, e.g., 25 wt % and the pH of the solution is from 10 to 13. Furthermore, standard reaction temperature is 100° C. or higher and reaction time is 2 days or more, and thus, it is synthesized using an autoclave. The porous body obtained by the hydrothermal synthesis has remarkably homogenous mesopore diameter as compared with the conventional porous materials and has a characteristic structure where the fine pores are regularly arranged.
In the case where such mesoporous porous bodies having a regular fine pore structure is applied to functional material fields other than catalysts, it is important to uniformly hold these materials on a substrate. As processes for producing a homogeneous mesoporous thin-film on a substrate, there may be mentioned a process by spin-coating as described in Non-Patent Document 4, a process by dip-coating as described in Non-Patent Document 5, a process of precipitating a film on a solid surface as described in Non-Patent Document 6, and the like. The thickness of the thin film provided on the substrate is usually about several micrometers.
There are known a silica mesostructure thin film formed on a polymer compound film provided on a substrate wherein the thin film is formed on part or all of the polymer compound film having structural anisotropy imparted on the surface by linear polarized light irradiation (Patent Document 2) and a method of maintaining alkaline pH in an aqueous solution containing a surfactant and heating the whole to remove the surfactant (Patent Document 3). A method by spin coating (Patent Document 4) and the like are known.
The mesoporous silica of the porous material has a characteristic that it has a regular arrangement such as lamella, hexagonal, or cubic one or, even when it has no regular arrangement, has homogeneous mesopores. Since it has a large pore volume and a large number of hydroxyl groups are present on the surface of the fine pore wall, absorbed amount of water is large. Thus, use as a water absorbent has been developed and applications to an absorbent for separation, a sensor, a support for catalysts, and a fuel cell have been investigated, as well as studies for producing a film thereof have been performed.
It has been found that the film has an excellent gas adsorbing property and also has a function of separating various substances. Therefore, a variety of processes for producing such a porous body having a void structure have been proposed.
Moreover, there are a number of reports as the other production processes. For example, as a process for producing mesoporous silica at normal temperature for a short period of time, a synthetic process using a vacuum evaporator or the like devised by Dr. Endo, researcher of National Institute of Advanced Industrial Science and Technology is described in Patent Document 5. The process possesses advantages that rapid synthesis is possible at low temperature as compared with the hydrothermal synthesis and operation is simple and cost-performance is excellent since a solid-liquid separating step and a washing step are unnecessary. Furthermore, the resulting porous material also possesses an advantage of high steam resistance.
The aforementioned mesoporous silica is expected as an adsorbent for water and organic vapors based on its uniform and regular fine pore structure. For example, in the case where it is considered as a water vapor adsorbent, since it exhibits a large adsorption and desorption amount in a specific narrow relative humidity range depending on a pore diameter and the adsorption is attributable to capillary condensation, it has a large potential as a novel adsorbent (moisture adsorbent) which requires small energy for regeneration, requires low temperature for regeneration, and has a large adsorption amount. Such adsorption properties are excellent properties which are not exhibited by conventionally often employed zeolites and silica gel.
At the time when an adsorbent is applied to an actual adsorption system (e.g., desiccant air conditioning), it is necessary to fix an adsorbent on a suitable substrate. The most common one is a honeycomb rotor and usually, a ceramic paper or the like is used. For supporting an adsorbent on the honeycomb rotor, it is necessary to carry out steps of dispersing the adsorbent into a solution to form a slurry with a binder, impregnating a parent material of the honeycomb rotor therewith, and then drying and calcining the same. Moreover, in the case where the adsorbent is used as an adsorbent for an adsorbtion heat pump, it is considered that it is desirable to fix the adsorbent on a metal fin from the viewpoint of improving heat-transfer properties.
Specifically, there is a known process for producing a mesoporous SiO2 thin-film having a three-dimensional structure, which includes obtaining an organo-inorgano composite SiO2 thin-film having a three-dimensional structure formed on the base material by mixing an alkylene oxide block copolymer and a tetraalkyl orthosilicate in an ethanol solution, effecting hydrolysis with adjusting the solution to a low pH region to form a sol solution, dropping the sol solution onto a substrate, rotating the substrate at a high speed, and evaporating the solvent to effect gelation; and subsequently sintering the thin-film (Patent Document 6) and the like process.
Since ordered mesoporous silica is regularly deposited, i.e., homogeneous pore channels are directed to a transverse direction in this kind of mesoporous material, in the case where such a mesoporous material is used for an insulating layer (low-k material) of a highly integrated electronic circuit, a stress is to be applied from a lateral side of the fine pore channels, i.e., from above during a processing step. This kind of the mesoporous material having a honeycomb structure is weak in mechanical strength of the lateral side of the fine pore channels. Therefore, a conventional mesoporous film is apt to be broken at the fine pores during the above processing step. Moreover, in the case of using the conventional mesoporous film as a separating film, since substances are permeated through inside of the fine pores and separated, the mesoporous material where the homogeneous pore channels are directed to a transverse direction cannot virtually be utilized as a separating film. This is also true in the case where it is utilized as a chemical sensor. Furthermore, in the case of using the mesoporous material as a high-density recording medium, it becomes possible to utilize it as a high-density recording medium only when individual fine pores act as recording units. Therefore, when the fine pore channels are directed to a transverse direction, reading and writing are difficult and effective surface area participating in recording is small, so that effects are difficult to be exhibited. From the above, in order to effectively apply the mesoporous body, it is desired to develop a material where the mesopore channels having homogeneous pores pierced vertically are regularly arranged. There is known a polysilicate salt having a structure where a silicate sheet having a six-membered ring is vertically arranged (Patent Document 7).
Moreover, there is known a process wherein (A) an anionic surfactant, (B) a silicate monomer, and (C) a basic silane are mixed in water or a mixed solvent of water and an organic solvent miscible therewith to obtain a mesoporous silica composite having homogeneously sized mesopores; the mesoporous silica composite is washed with an acidic aqueous solution or an organic solvent miscible with water or an aqueous solution thereof to remove the anionic surfactant of the component (A), thereby obtaining a mesoporous silica outer shell having the structure of the mesoporous silica composite as a template; and the mesoporous silica composite or the mesoporous silica outer shell is calcined (Patent Document 8).
However, the conventionally known mesoporous silica film is limited to a μm-order thin-film and no thick-film has been produced yet.
For obtaining a mesoporous silica thick-film, although it may be considered that the film thickness may be adjusted by a direct dip coating method or the like onto a substrate, actually, the film cannot be formed in an arranged state having a regular structure, so that it becomes difficult to form a thick-film.
If a thick-film is obtained as compared with the conventional thin-films, mesoporous silica is expected to be as an adsorbent for water and organic vapors owing to its homogeneous and ordered mesopore structure. For example, in the case where it is considered as a water vapor adsorbent, since it exhibits a large adsorption and desorption amount in a specific narrow relative humidity range depending on a mesopore diameter and the adsorption is attributable to capillary condensation, it has a large potential as a novel adsorbent (moisture absorbent) which requires small energy for regeneration, requires low temperature for regeneration, and has a large adsorption amount. Owing to the adsorbing properties, it becomes possible to obtain a film which allows development of an air-cleaning system having excellent properties which are not exhibited by conventionally often employed zeolites and silica gel.
Thus, development of a mesoporous silica thick-film has been highly desired.
Patent Document 1: WO91/11390
Patent Document 2: JP-A-2002-338229
Patent Document 3: JP-A-2004-27270
Patent Document 4: JP-A-2002-250713
Patent Document 5: Japanese Patent Application No. 2003-385662
Patent Document 6: JP-A-2002-250713
Patent Document 7: JP-A-2003-335516
Patent Document 8: JP-A-2004-345895
Non-Patent Document 1: Nature. Vol. 359, p. 710
Non-Patent Document 2: Journal of Chemical Society Communications. Vol. 1993, p. 680
Non-Patent Document 3: J. Am. Chem. Soc. 11410834 (1992)
Non-Patent Document 4: Chemical Communications. Vol. 1996, p. 1149
Non-Patent Document 5: Nature. Vol. 389, p. 364
Non-Patent Document 6: Nature. Vol. 379, p. 703